I'm just guessing here, but chances are if these students aren't taking hits well, they are perhaps flinching, turning away, or backpedalling excessively. When people lack confidence, they tend to do these things, and as a result, when a shot gets through, they often don't see it coming which makes it harder to absorb.

Attack/defence drills are always good for that. "Toe the line" is my favorite. - two people stand in range of each other, with pieces of tape on the floor marking where they must each put their front foot. They then spend a round each as an aggressor or defender, firing off punches at whatever pace is comfortable, with the defender not allowed to move their front foot from the line. The second they step off the line the action stops until they resume their starting position.

You can build up the intensity of the activity over time, at a pace that suits the individual.